The objective of this study was to establish conditions for the flow cytometric analysis of cytokine production and to analyze whether there are differences in cytokine production that might contribute to differences in pathogenicity of SIV infection between rhesus macaques and sooty mangabeys. Optimal conditions were established for the flow cytometric detection of cytokine production in rhesus macaques and sooty mangabeys. Cytokine production in PBMC was assessed by flow cytometry following surface staining with CD4 and CD8-specific antibodies and intracellular staining of cytokines using antibodies for the cytokines IL-2, IL-4, IL-10, g-interferon and TNF-a. Cytokine production was assessed on fresh PBMC without in vitro stimulation. One rhesus macaque and one sooty mangabey were studied prospectively following infection with SIVmac239. There were no differences at baseline between the animals. However, at day 7 following SIV infection, an increase in IL-10 production was observed in the sooty mangabey while there was a preponderance of IL-2 and g-interferon production in the macaques. The animals have been followed up at monthly intervals for five months. The macaque continued to show a predominant Th1 type of cytokine secretion pattern. In contrast, the sooty cells expressed have intermittently displayed a Th2 type of cytokine secretion